1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a computer-readable recording medium. More particularly, the present invention relates to an image processing apparatus which restrains interference between a pattern and dithering, an image processing method, and a computer-readable recording medium.
2. Description of the Related Art
In printing a document or image data by an image processing apparatus such as a printer, bitmap data corresponding to the document or image data is used. Such an image processing apparatus performs gradation expression through dithering. However, there has been a problem that, when dithering is applied to data at a minute pattern portion such as a checkerboard pattern in a document or an image, an output result is disrupted by interference between the pattern and dithering.
In recent years, with the improvement in performance of information processing apparatuses such as personal computers, functions of applications have been diversified increasing the use of transparent expressions in addition to patterns and filling expressions. However, at the time of printing, even an object transparently expressed on an application may be transparently expressed in a pseudo way by using a pattern in some cases because of a restriction on an interface between a computer and a printer unit. Therefore, in recent years, there has been a trend toward increasing use of print data including a pattern, and accordingly interference between the pattern and dithering has become a serious problem.
There is a technique for digitally correcting print data to correct misalignment caused by bends and inclinations of a laser beam in a printer unit including a laser scanner. This technique eliminates a process for optically adjusting a laser scanner unit. This technique has a problem that interference between a pattern and dithering disturbs digital correction resulting in color unevenness.
In digital correction, in consideration of bends and inclinations of the laser beam, print data (bitmap image) is corrected by shifting in a subscanning direction on a pixel basis in advance by an amount of distortion in an image produced by bends and inclinations of the laser beam.
Further, to reduce a level difference produced by misalignment at a boundary, interpolation processing is applied for correction under pixel. In this correction processing, interpolation is performed at a portion where a level difference is influential (visually noticeable), such as a character and line, and not performed at a portion where a level difference is not influential (visually unnoticeable) since interpolation will cause uneven density. In this way, the processing determines whether or not interpolation is to be performed depending on the type of image data.
However, when an image including interference between a pattern and dithering is used to determine a character, a line, and a gradation image, a change in density is emphasized by the interference and accordingly an image portion that should originally be determined to be a gradation image may be incorrectly determined as a character or a line. As a result, interpolation processing performed based on the incorrect result of determination may cause color (density) unevenness at a gradation image portion.
A technique discussed in Japanese Patent Application Laid-Open No. 2008-23960 analyzes a rendering command included in print data and, when filling with a pattern is specified, converts the relevant command into multi-value data to enable filling with an even density. In particular, data is expressed by an average gradation when filling with an overwrite pattern is specified and by transmittance when filling with a mask pattern is specified. This prevents interference between the pattern and dithering.
The above described conventional method, however, converts data into an average gradation or transmittance when filling with a pattern is specified. With this method, the following two different checkerboard patterns are converted into an average gradation or transmittance. One is a checkerboard pattern composed of minute ON and OFF dots preferably intended to be converted into an average gradation or transmittance, and the other is a checkerboard pattern composed of large ON and OFF dots to some extent not intended to be converted thereinto. In this case, although interference does not occur, an image in a print result may be visually different from what is intended by a user.
Further, since a pattern to be transparently expressed in a pseudo way is expressed with a combination of specific rendering commands, a pattern to be transparently expressed in a pseudo way can be determined by analyzing each rendering command. However, in the determination based only on the combination of rendering commands, there may be a situation that even a pattern other than ones transparently expressed in a pseudo way will be incorrectly determined to be a pattern to be transparently expressed in a pseudo way. Therefore, a pattern that should not originally be converted into a transparent expression may be unnecessarily converted thereinto.